Amidation is a fundamentally important synthetic process in chemistry and biology. The amide group, an important motif in natural products, polymers, and pharmaceuticals, may be introduced by reactions of carboxylic acids or their derivatives with amines, modified Staudinger reactions, aminocarbonylation of halides with CO and amines, rearrangements of ketoximes and aldoximes, and transition-metal-catalyzed carbonylation of alkenes and alkynes with amines. These processes frequently involve complex procedures and costly or hazardous reagents, and may produce toxic chemical waste.
The direct oxidative synthesis of amides via catalyzed reactions of amines with alcohols or aldehydes has recently attracted interest, because of the environmentally-benign nature of the reactions and the wide availability of the starting materials. A few homogeneous transition metal complexes have been reported to catalyze this class of reactions effectively, although the mechanistic details of the reactions remain unclear. (D. Gunanathan et al., Science 2007, 317, 790; L. U. Nordstrøm et al., J. Am. Chem. Soc. 2008, 130, 17672; W. Yoo, and C Li, J. Am. Chem. Soc. 2006, 128, 13064; S. Y. Seo and T. J. Marks, Org. Lett. 2008, 10, 317.)
For industrial-scale syntheses, heterogeneous catalysts are much preferred, and recent studies of supported gold and silver catalysts in the direct synthesis of amides suggest that heterogeneous processes may be possible. (S. K. Klitgaard et al., Green Chem. 2008, 10, 419; T. Ishida and M. Haruta, Chemsuschem 2009, 2, 538; K. Shimizu et al., Chem. Eur. J. 2009, 15, 9977.)
The prior art processes for this transformation are handicapped by certain disadvantages, such as high pressures, high temperatures, organic by-products, and soluble and/or expensive catalysts. There remains a need for efficient, cost-effective, heterogeneous catalytic syntheses of amides.